Nuclear spin conversion of molecular hydrogen on amorphous solid water in the presence of O2 traces

Nuclear spin conversion (NSC) of ortho- to para-H(2) and para- to ortho-D(2) has been investigated on an amorphous solid water (ASW) surface at 10 K, in the presence of co-adsorbed O(2). The dynamics of the nuclear spin conversion could be revealed by combination of resonance enhanced multiphoton ionization spectroscopy (REMPI) with temperature programmed desorption (TPD) experiments. The conversion rates are consistent with a diffusion of molecular hydrogen inducing a nuclear spin conversion enhanced in the vicinity of molecular oxygen. The conversion times were found to increase with decreasing O(2) and H(2) coverage. Finally, on oxygen free ASW surface, the extremely long conversion characteristic times measured showed that such surface is not an efficient catalyst for NSC, in contradiction with hypothesis commonly made for interstellar medium.     

Catalytic activity of tantalum hydride in parahydrogen conversion

The kinetics of parahydrogen conversion were studied on tantalum hydride at a hydrogen pressure of 2 Torr, in the temperature range 20–100 °C. For samples with high hydrogen content (H/Ta>0.7) the Arrhenius plot shows a distinct break at 52 °C near to the order-disorder transition temperature. For the ordered -Ta hydride phase the activation energy and preexponential factor are lower than those for the disordered phase of the Ta-H system.

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- 2 20–100°C. (H/Ta>0,7) 52°C -. - Ta- - Ta-H.

     

Rotation of water in solid parahydrogen and orthodeuterium

The far-infrared spectra of solid orthodeuterium and solid normal deuterium are presented and compared to the corresponding spectra of solid parahydrogen and solid normal hydrogen. Spectra of water in orthodeuterium are compared to spectra of water in parahydrogen. The water rotation constants in orthodeuterium are approximately 80% of the rotation constants of water in parahydrogen. The S(0)(0) band of orthodeuterium gets a strong satellite in the presence of water. The position and width of the satellite depends on the isotopic composition of the water present. If there is a corresponding satellite in parahydrogen it is weak and closer to the S(0)(0) band of the matrix. The conclusion of the paper is that interaction between guest rotation and the rotation of matrix molecules must be taken into account to explain the reduction of the rotation constants in orthodeuterium.     

Infrared-induced reaction of Cl atoms trapped in solid parahydrogen

The 355 nm photodissociation of Cl(2) trapped in a solid parahydrogen matrix at 2 K leads to the formation of isolated Cl photofragments. At these low temperatures (k(B)T approximately 1.4 cm(-1)), the Cl atoms can not react with the parahydrogen matrix since the reaction Cl + H(2)(v = 0, j = 0) --> HCl(v = 0, j = 0) + H is endothermic by 360 cm(-1). Irradiation of the Cl atom doped parahydrogen solid with broadband infrared radiation from 4000 cm(-1) to 5000 cm(-1) induces reaction of atomic Cl with the parahydrogen matrix to form HCl. The infrared-induced chemistry is attributed to solid parahydrogen absorptions that lead to the creation of vibrationally excited H(2)(v = 1), which supply the necessary energy to induce reaction. The kinetics of this low temperature infrared-induced reaction is studied using Fourier Transform infrared spectroscopy of the HCl reaction product. The HCl formation kinetics is first-order and the magnitude of the effective rate constant for the infrared-induced reaction depends on the properties of the near infrared radiation.     

Spectroscopy of HF and HF-containing clusters in solid parahydrogen

We report measurements of FT-IR absorption spectroscopy of HF, DF, and their clusters in solid parahydrogen (pH(2)). The observed spectra contain many absorption lines which were assigned to HF monomers, HF polymers, and clusters with other species, such as N(2), O(2), orthohydrogen (oH(2)), etc. The rotational constants of HF and DF monomers were determined from the cooperative transitions of the vibration of solid pH(2) and the rotation of HF and DF. Small reduction of the rotational constants indicates that HF and DF are nearly free rotors in solid pH(2). Time dependence of the spectra suggests that HF and DF monomers migrate in solid pH(2) and form larger polymers, probably via tunneling reactions through high energy barriers on inserting another monomer to the polymers. The line width of HF monomers in solid pH(2) was found to be 4 cm(-1), which is larger than that of other hydrogen halides in solid pH(2). This broad line width is explained by rapid rotational relaxation due to the accidental coincidence between the rotational energy of HF and the phonon energy with maximum density of states of solid pH(2) and the rotational-translational coupling in a trapping site.     

Density of phonon states in solid parahydrogen from inelastic neutron scattering

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..     

Photocatalytic conversion of methane

Various efforts have been carried out to convert methane to more useful chemicals and hydrogen. However, due to its high stability, high energy is usually consumed for its conversion, which still remains as a problem to be solved. Recently, photocatalysis has been proposed to be one of the answers to break the thermodynamic barrier. This tutorial review provides a brief history about developments in the methane conversion and specially highlights the developments in the photocatalytic conversion of methane, such as methane coupling and methane conversion with other molecules.     

Crystal field splitting of rovibrational transitions of water monomers isolated in solid parahydrogen

We report polarized infrared absorption spectra of water isotopologues isolated in solid parahydrogen (pH2) which reveal the crystal field induced splittings of the 1 01<--0 00 R(0) lines in the nu1 HDO, nu3 D2O, nu3 HDO, and nu3 H2O fundamental bands. For annealed pH2 solids, these spectra also reveal a strong alignment of the hexagonal-close-packed crystallites' c axes with the deposition substrate surface normal. This alignment effect explains our failure to detect the parallel-polarized components of these R(0) lines in spectra of pH2 solids produced on a transparent deposition substrate [M. E. Fajardo et al., J. Mol. Struct. 695, 111 (2004)]. This lesson applies more generally to comparison of solid pH2 spectra obtained in different laboratories. The spectra are consistent with water monomers existing in solid pH2 as very slightly hindered rotors. The individual components of the R(0) absorption lines show a Lorentzian lineshape, with vibrational depopulation the most important source of line broadening.     

Nuclear spin conversion to probe the methyl rotation effect on hydrogen-bond and vibrational dynamics

A noteworthy example of a molecule with coupled large-amplitude motions is provided by acetylacetone (methyl group torsions and intramolecular hydrogen bonds). The molecule was trapped in solid parahydrogen to investigate the complex proton tunneling processes. Nuclear spin conversion in methyl groups is observed and, combined with IR spectra, documents the coupling between high frequency modes and large amplitude motions.     

High-resolution spectroscopy and the analysis of ro-vibrational transitions of molecules in solid parahydrogen

Solid parahydrogen provides a novel matrix for isolation spectroscopy of atoms and molecules. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interactions in the crystal and of the softness of the crystal being characteristic of quantum crystals. Most of the observed spectral linewidths are one or two orders of magnitude sharper than those observed in conventional rare gas matrices. The sharp linewidths make the parahydrogen crystal an excellent matrix for the study of ro-vibrational states and dynamics of dopant molecules in the condensed phase by high-resolution spectroscopy. In this article, we have summarized the most fundamental part of our study, that is, the theory of ro-vibrational states of dopant molecules in the crystal, which is necessary for the quantitative analysis of high-resolution infrared spectra. We also discuss what we have learned from the analysis of high-resolution infrared spectra in solid parahydrogen. These include perturbations to rotational motion of dopant molecules, reduction of rotational constants, vibrational dephasing and relaxation. Outstanding questions to be solved are also discussed herein.     

Synthesis and infrared characterization of Br-HBr and Br-DBr entrance channel complexes in solid parahydrogen

We report high resolution vibrational spectra in the HBr (2560 cm(-1)) and DBr (1840 cm(-1)) stretching regions for Br-HBr and Br-DBr entrance channel complexes isolated in solid parahydrogen (pH2). The Br-HBr complexes are generated by synthesizing solid pH2 crystals doped with trace amounts of HBr/Br2 mixtures followed by 355 nm in situ photodissociation of Br2 to form Br atoms. After photolysis is complete, the solid is warmed from 2 to 4.3 K resulting in the irreversible formation of Br-HBr complexes. The large 36.63 cm(-1) HBr monomer-to-complex induced vibrational shift to lower energy measured in these studies is consistent with the linear Br-HBr hydrogen bonded structure predicted from theory. The 0.02 cm(-1) Br-HBr absorption linewidths indicate a 1 ns vibrational excited state lifetime for these entrance channel complexes in solid pH2.     

Nuclear spin polarization in radical reactions

An extension of the Closs-Kaptein-Oosterhoff theory concerning nuclear spin polarization resulting during free radical reactions is presented. This extension is based on the Merrifield model for the magnetic field dependence of triplet-triplet annihilation.     

Nuclear spin catalysis in living nature

Experiments with cells enriched in stable magnesium isotopes, magnetic ²⁵Mg or nonmagnetic ²⁴Mg and ²⁶Mg, are carried out. It is revealed that adaptation of bacteria E. coli to the growth media enriched in magnetic ²⁵Mg proceeds faster as compared to the growth media enriched in nonmagnetic magnesium isotopes. In experiments with another commonly accepted cell model, S. cerevisiae yeast, it is revealed that the rate constant of postradiation recovery of the cells after UV irradiation is two times higher for cells enriched in ²⁵Mg than for cells enriched in the nonmagnetic isotope. In collaboration with Ukrainian colleagues from the Palladin Institute of Biochemistry, the effects of different isotopes of magnesium on ATPase activity of myosin isolated from myometrium are studied. It is found that the rate of the enzymatic hydrolysis of ATP for ²⁵Mg is 2.0–2.5 times higher as compared to nonmagnetic isotopes ²⁴Mg and ²⁶Mg. Some possible mechanisms of magnetic isotope effects (nuclear spin catalysis) in biological objects are discussed.     

Nuclear spin catalysis in biochemical physics

Magnetic isotope effects have been recently discovered in living nature. They were observed for the first time in experiments on cells enriched with various magnesium isotopes, magnetic ²⁵Mg or non-magnetic ones. A catalytic effect of the magnetic isotope of magnesium was discovered in experiments with myosin, the most important biomolecular motor utilizing the energy of ATP to perform mechanical work. The rate of ATP hydrolysis with the magnetic ²⁵Mg isotope is 2.0–2.5 times higher than that obtained with nonmagnetic ²⁴Mg or ²⁶Mg. A similar effect of the nuclear spin catalysis was experimentally observed for zinc isotopes. The rate of ATP hydrolysis in the case of magnetic ⁶⁷Zn increased by 40–50% as compared to that observed experimentally for nonmagnetic isotopes (⁶⁴Zn or ⁶⁸Zn). Catalytic effects of the magnetic isotope of magnesium were also experimentally found for H⁺-ATPase isolated from yeast mitochondria and ATPase of the plasma membrane of the myometrium. The magnetic isotope effect indicates unambiguously that the chemomechanical processes involve a limiting step catalyzed by biomolecular motors, which depends on the electronic spin state, and that this step is accelerated in the presence of nuclear spin of the magnetic isotope.

     

Effect of density on the induced near infrared absorption spectrum of solid parahydrogen

The evolution of the induced near infrared absorption spectrum of solid parahydrogen as a function of pressure, at 4.2 K and up to 10 kbar, is given here. A blue frequency shift and a strong intensity enhancement are observed, which depend upon the nature of the line. The continuous presence of an S₁(0) line shows that molecules are not situated at centers of inversion symmetry, and it appears that the crystalline structure remains hexagonal close-packed (hep) in the compressed solid.     

Electron spin resonance study on H6+, H5D+, H4D2+, and H2D4+ in solid parahydrogen

We carried out an electron spin resonance (ESR) study on hydrogen ion radicals produced by radiolysis of solid para-H(2). In addition to quartet ESR lines proposed to be H(2) (+)-core H(6) (+) (D(2d)) ions in solid para-H(2) [T. Kumada et al., Phys. Chem. Chem. Phys. 7, 776 (2005)], we newly observed totally more than 50 resolved lines in gamma-ray irradiated solid para-H(2)-ortho-D(2) (1 mol %) and para-H(2)-HD (1 mol %) mixtures. We assigned these lines to be isotope substituents of H(2) (+)-core H(6) (+) ions such as H(5)D(+), H(4)D(2) (+), and H(2)D(4) (+) throughout the comparison of their ESR parameters with theoretical results. These results provide a conclusive evidence that H(2) (+)-core H(6) (+) ions are generated in irradiated solid hydrogens. Analysis of the EPR spectrum and ab initio calculations predicts D(2d) symmetry of the H(6) (+) ions, whereas a lowering symmetry (D(2d)-->C(2v)) induced by asymmetric nuclear wave function is observed in H(5)D(+) and H(4)D(2) (+). We also observed isotope-substitution reactions such as H(6) (+)+D(2)-->H(4)D(2) (+)+H(2) and H(6) (+)+HD-->H(5)D(+)+H(2), which are analogous to the well-known isotope-condensation reactions of H(3) (+) in dark nebula, H(3) (+)+HD-->HD(2) (+)+H(2) and HD(2) (+)+HD-->D(3) (+)+H(2).     

Biological conversion of coal gas to methane

Biological conversion of low-Btu coal synthesis gas to higher Btu methane was demonstrated using both pure co-cultures and/or adapted-mixed anaerobic bacteria.Peptostreptococcus productus metabolized coal gas to mainly acetate and CO₂. The co-cultures containing methanogens converted these products to methane. In mixed culture studies, CH₄ and small amounts of acetate were produced. Reactor studies using stirred-tank and immobilized cell reactors exhibited excellent potential to convert CO, CO₂ and H₂ to methane at higher gas flow rates. Gas retention times ranging from 0.7 to 2 hours and high agitation were required for 90 percent CO conversion in these systems. This paper also illustrates the potential of biological methanation and demonstrates the need for good mass transfer in converting gas phase substrates.     

单原子催化甲烷直接转化的研究进展

将甲烷直接转化(DMC)为高附加值化学品(如甲醇等化合物),是实现天然气高效利用的有效途径.因甲烷结构非常稳定,使其在温和条件下(反应温度≤150℃的非强酸介质体系)的高效活化极具挑战性.近年来,单原子催化剂(SACs)因其活性物种的高利用率和高选择性,已引起国内外研究者的广泛关注,并被尝试应用于多种反应.研究表明,S...     

Microbial conversion of high-rank coals to methane

Applied Biochemistry and Biotechnology - It has been demonstrated recently that certain bacteria and fungi are capable of directly or indirectly converting low-rank coals into liquid and gaseous...